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Re: #227: HEAD and Caches

From: Henrik Nordström <henrik@henriknordstrom.net>
Date: Thu, 16 Feb 2012 08:22:18 +0100
Message-ID: <1329376938.22776.20.camel@home.hno.se>
To: Mark Nottingham <mnot@mnot.net>
Cc: "ietf-http-wg@w3.org Group" <ietf-http-wg@w3.org>
tis 2012-02-07 klockan 15:03 +1100 skrev Mark Nottingham:

> A few problems:
> 1. Since it specifies required cache behaviour, it really should be in p6


> 2. The second MAY seems to conflict with the MUST

Not really but should perhaps read "cached", not "cache".

It's a mutually exclusive condition. Remember that MUST have priority
over MAY, and with the two even in the same sentence there isn't much
room for confusion.

If the cached entry identity matches the HEAD response then it may be
updated with information from HEAD just as in the case of 304 responses
or 204 responses.

If the cached entry identity do not match the HEAD response then the
cached entry is stale even if it's cached expiry information says

> 3. Caches can store multiple representations for a resource, so there is no "current representation."

Not really a problem.

If there is no Vary then URI == Resource.

On resources using Vary then representations of URIs should have
identification means making them unique

a) ETag
b) Content-Location

and on Vary not using * then there is a clear mapping of
request->representation allowing HEAD responses to clearly indicate
earlier responses that match this request as stale.

> Part of the problem here really is that it's not "updating" any
> response, but it is potentially invalidating an old one.


HEAD responses that match the cached representation may update the
cached representation with new Date etc. It's a form of cache validation
if you like.

> To resolve this, we could construct a requirement that refers to p6
> 2.7 ("Caching Negotiated Responses") to identify the correct response
> to compare to and (potentially) invalidate. 

It's simply the last known response that matches the request.

This response may be composed by aggregating many earlier responses

HEAD response
merged 204 responses
partial 200 response
200 response

> However, I wonder if a) this is widely implemented, and b) the
> complexity is worth it.

a) Probably not.

b) Probably worthwhile explaining the above response merge model of

> I.e., we could alternatively just remove everything after the first
> sentence (i.e., treat the second MAY as primary, and therefore make
> the whole thing redundant).

Some common guidance on how/when cache entries are
updated/merged/invalidated may help. The same properties & conditions
applies in all cases. Both in intent (keeping cache up to date avoiding
unneeded roundtrips) and issues (avoiding bad updates, servers not
providing correct data in many cases)

Received on Friday, 17 February 2012 01:38:08 UTC

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